Waveguide grating routers in conventional lightwave systems are used as optical switches, multiplexers, demultiplexers, detectors, add/drop filters, one by N (1.times.N) and N by one (N.times.1) splitters and N by N (N.times.N) arrays. Typically, such waveguide grating routers include an interconnection apparatus having a plurality of closely spaced input waveguides communicating with an input of a star coupler. An output of the star coupler communicates with an optical grating comprising a series of optical waveguides in which each of the grating waveguides differ in length with respect to its nearest neighbor by a predetermined fixed amount. The optical grating is further connected to an input of a second star coupler, the outputs of which form outputs of the switching, multiplexing, and demultiplexing apparatus. Waveguide grating routers are also frequently referred to as "frequency routing devices" and are further described in U.S. Pat. No. 5,002,350, issued Mar. 26, 1991 to C. Dragone, entitled "Optical Multiplexer/Demultiplexer" (hereinafter "Dragone 1") and U.S. Pat. No. 5, 136,671, issued Aug. 4, 1992 to C. Dragone, entitled "Optical Switch, Multiplexer, and Demultiplexer" (hereinafter "Dragone 2"), both of which are hereby incorporated by reference.
A known characteristic of these waveguide grating routers is that they do not efficiently provide a predetermined composite amplitude spectrum for each of the output signals that may comprise many wavelengths or frequencies. Rather, these routers tend to attenuate signals in outer waveguides more than signals in the inner waveguides. Thus, the amplitude response of the router across the spectrum of wavelengths in the waveguides is nonuniform.
Attempts have been made to design a waveguide grating router with a uniform composite amplitude spectrum, yet prior designs have proven to be inefficient in deriving a predetermined composite amplitude spectrum, such as a uniform output spectrum, or such designs suffer severe loss. For example, one method uses a "loop-back" optical path with the waveguide grating router. "Loop-back" optical path connections are connections from the output signal fed back into a predetermined input port as described in an article entitled "Loss Imbalance Equalization of Arrayed Waveguide Grating Add-Drop Multiplexer," written by Osamu Ishida, et. al., that appeared in Electronics Letters, Vol. 30, No. 14, Jul. 7, 1994. However, this method has not been effective to equalize loss in waveguide routers.
Another method used for achieving a uniform output spectrum averages the waveguide grating router loss over the cascaded routers by shifting the port connections between adjacent routers, as described in an article entitled "Loss Imbalance Equalization in Arrayed Waveguide Grating (AWG) Multiplexer Cascades" written by Osamu Ishida, et al., that appeared in Journal of Lightwave Technology, Vol. 13, No. 6, Jun. 1995. However, such a system suffers severe loss due to the cascading of routers.
Still another method to achieve greater channel uniformity was described in an article entitled "Waveguide Grating Router with Greater Channel Uniformity" written by J. C. Chen and C. Dragone, that appeared in Electronics Letters, Vol. 33, No. 23, pp. 1951-2, Nov. 6, 1997. The method disclosed therein, which is incorporated by reference herein, uses auxiliary waveguides between adjacent grating arms in a waveguide grating router. Although such a system does not add loss, it also does not provide a predetermined composite amplitude spectrum, such as a uniform output spectrum. Specifically, the auxiliary waveguides simply help to shape the radiation pattern of the star coupler within the waveguide grating router to reduce channel non-uniformity. Thus, there is a need for a more effective means of providing a waveguide grating router having a predetermined composite amplitude spectrum thereby reducing the complexity of lightwave systems.